Relationships Between Root Pathogen Resistance, Abundance and Expression of \u3ci\u3ePseudomonas\u3c/i\u3e Antimicrobial Genes, and Soil Properties in Representative Swiss Agricultural Soils

Strains of Pseudomonas that produce antimicrobial metabolites and control soilborne plant diseases have often been isolated from soils defined as disease-suppressive, i.e., soils, in which specific plant pathogens are present, but plants show no or reduced disease symptoms. Moreover, it is assumed that pseudomonads producing antimicrobial compounds such as 2,4-diacetylphloroglucinol (DAPG) or phenazines (PHZ) contribute to the specific disease resistance of suppressive soils. However, pseudomonads producing antimicrobial metabolites are also present in soils that are conducive to disease. Currently, it is still unknown whether and to which extent the abundance of antimicrobials-producing pseudomonads is related to the general disease resistance of common agricultural soils. Moreover, virtually nothing is known about the conditions under which pseudomonads express antimicrobial genes in agricultural field soils. We present here results of the first side-by-side comparison of 10 representative Swiss agricultural soils with a cereal-oriented cropping history for (i) the resistance against two soilborne pathogens, (ii) the abundance of Pseudomonas bacteria harboring genes involved in the biosynthesis of the antimicrobials DAPG, PHZ, and pyrrolnitrin on roots of wheat, and (iii) the ability to support the expression of these genes on the roots. Our study revealed that the level of soil disease resistance strongly depends on the type of pathogen, e.g., soils that are highly resistant to Gaeumannomyces tritici often are highly susceptible to Pythium ultimum and vice versa. There was no significant correlation between the disease resistance of the soils, the abundance of Pseudomonas bacteria carrying DAPG, PHZ, and pyrrolnitrin biosynthetic genes, and the ability of the soils to support the expression of the antimicrobial genes. Correlation analyses indicated that certain soil factors such as silt, clay, and some macro- and micronutrients influence both the abundance and the expression of the antimicrobial genes. Taken together, the results of this study suggests that pseudomonads producing DAPG, PHZ, or pyrrolnitrin are present and abundant in Swiss agricultural soils and that the soils support the expression of the respective biosynthetic genes in these bacteria to various degrees. The precise role that these pseudomonads play in the general disease resistance of the investigated agricultural soils remains elusive

Conservation tillage and organic farming induce minor variations in Pseudomonas abundance, their antimicrobial function and soil disease resistance

Conservation tillage and organic farming are strategies used worldwide to preserve the stability and fertility of soils. While positive effects on soil structure have been extensively reported, the effects on specific root- and soil-associated microorganisms are less known. The aim of this study was to investigate how conservation tillage and organic farming influence the frequency and activity of plant-beneficial pseudomonads. Amplicon sequencing using the 16S rRNA gene revealed that Pseudomonas is among the most abundant bacterial taxa in the root microbiome of field-grown wheat, independent of agronomical practices. However, pseudomonads carrying genes required for the biosynthesis of specific antimicrobial compounds were enriched in samples from conventionally farmed plots without tillage. In contrast, disease resistance tests indicated that soil from conventional no tillage plots is less resistant to the soilborne pathogen Pythium ultimum compared to soil from organic reduced tillage plots, which exhibited the highest resistance of all compared cropping systems. Reporter strain-based gene expression assays did not reveal any differences in Pseudomonas antimicrobial gene expression between soils from different cropping systems. Our results suggest that plant-beneficial pseudomonads can be favoured by certain soil cropping systems, but soil resistance against plant diseases is likely determined by a multitude of biotic factors in addition to Pseudomonas